Free volume study of amorphous polymers detected by solid-state C-13 NMR linewidth experiments

Temperature dependence of solid-state C-13 nuclear magnetic resonance linew idths of six different amorphous polymers was examined around their glass-t ransition temperatures (T-g) under the conditions of magic-angle spinning a nd high-power proton dipolar decoupling (DD). The observed temperature depe ndence was explained by motional averaging over a dispersion of isotropic c hemical shifts and an interference between local anisotropic motion and DD. On the assumption that the correlation time of the local anisotropic motio n follows the Williams-Landel-Ferry equation, we found that the reference t emperature approximately equals T-g+5 K. From this reference temperature, t he fractional free volume of amorphous polymers at T-g is estimated to be 4 .7%, which is much larger than that obtained from rheological experiments ( 2.5%). The difference is attributed to differences in the amplitude of moti ons detected by both measurements. (C) 2001 American Institute of Physics.